How to generate a fake top lighting effect?
Lighting effect aren't recommended for PBR games, but it can be useful for games running on old hardware or mobile games to save calculation by displaying the assets in shadeless/unshaded.


To create this shader, add an emissive shader on your high poly.
Use an Input > Texture Coordinate and use the Normal input. This Input colors the faces depending on their orientation relative to the world (this means the rotation must be applied with Ctrl + a).
Connect it to a Converter > Separate RGB and use the Blue channel.
By default only the bright values are visible, to display the dark values add a Color > MixRGB and set it to Screen.
You can add a fake Global Illumination effect by adding another Screen node.
To bake your texture on the low poly use the Emit bake mode. You will find more informations about baking with Cycles here: https://blender.stackexchange.com/a/13509/23134
(optional) Blender's viewport and tools work in sRGB. If you want to export it to work in Photoshop or GIMP you will need to convert it to Linear, for that add a Color > Gamma and set the gamma value to 2.2.